Of many devices available for the detection of smoke, the projected-beam type of smoke detector has several advantages over the more common types of smoke detector in which smoke is sensed within a chamber. A projected-beam detector provides very high sensitivity to even low smoke densities because of its ability to integrate over a relatively large volume of smoke; it provides the capability for covering a large area per device; and it provides quick response time because of the lack of resistance to smoke admittance which is inherent to chamber-type smoke detectors.
In spite of these advantages, projected-beam systems have disadvantages which have prevented their widespread acceptance. Projected-beam detectors are sensitive to alignment and must be carefully aimed on installation. Additionally, projected-beam detectors are more prone to false alarms caused by accidental beam blockage, such as by carelessly carried ladders, moths, or large buildups of dust on the optical components thereof.